With progress in miniaturization, it has become feasible recently to use a gate length of 0.1 μm or less in an ultrahigh speed semiconductor device. Generally, an operational speed of a semiconductor device is improved with miniaturization, while in such a highly miniaturized semiconductor device, the thickness of a gate insulating film needs to be reduced in accordance with a scaling law with the miniaturization, in addition to a reduction in the gate length. Thus, in case the gate length is reduced to 0.1 μm or less, it is necessary to set the thickness of the gate insulating film to 1–2 nm or smaller when a conventional silicon thermal oxide film is used for the gate insulating film. In such an extremely thin gate insulating film, a tunneling current is increased, which in turn inevitably increases a gate leakage current.
Under such a situation, there has been a proposal of using as the gate insulating film a high-k dielectric material, such as Ta2O5, Al2O3, ZrO2, HfO2, ZrSiO4 or HfSiO4, having a dielectric constant much larger than that of the conventional silicon thermal oxide film, whose film thickness will become small when converted into a silicon thermal oxide film despite a large actual film thickness.
In a semiconductor device that uses such a high-k dielectric film for the gate insulating film, it is preferable to form the high-k dielectric film directly on a silicon substrate in order to reduce an effective thickness of the insulating film converted into the silicon thermal oxide film. However, in the case of forming the high-k dielectric film directly on the silicon substrate, metal elements in the high-k dielectric film tend to diffuse into the silicon substrate to thereby cause a carrier scattering problem in a channel region.
From the viewpoint of improving carrier mobility in the channel region, it is preferable to interpose an extremely thin base oxide film of a thickness of 1 nm or less, preferably 0.8 nm or less, between the high-k dielectric gate oxide film and the silicon substrate. Such an extremely thin base oxide film has to cover the surface of the silicon substrate uniformly, without forming defects such as interface states.
Conventionally, a thin gate oxide film used to be formed by a rapid thermal oxidation (RTO) on a silicon substrate. When forming a thermal oxide film of a desired thickness of 1 nm or less, it is necessary to reduce a processing temperature used at the time of film formation. However, the thermal oxide film formed at a low temperature is liable to include defects such as the interface states and is deemed inappropriate to be used for the base oxide film of the high-k dielectric gate oxide film.
Therefore, in forming a base oxide film, the inventors of the present invention have previously proposed to use a UV-excited oxygen radical (UV-O2 radical) substrate processing unit capable of forming a high-quality oxide film at a low film forming speed based on a low radical density (see Japanese Patent Laid-open Application No. 2002-100627).
FIG. 21 shows a schematic configuration of a conventional UV-O2 radical substrate processing unit 100. Referring to FIG. 21, the substrate processing unit 100 includes a processing chamber 101 for keeping a substrate 102 under a depressurized environment, wherein the substrate 102 to be processed is held on a susceptor 101A provided with a heater 101a. Further, there is provided a shower head 101B in the processing chamber 101 which is arranged to face the substrate 102 held on the susceptor 101A, and an oxidizing gas, such as an oxygen gas, O3, N2O, NO or a mixture thereof, is supplied to the shower head 101B.
The shower head 101B is formed of a material transparent to an ultraviolet light such as quartz, and there is provided a window 101C, formed of quartz and the like, for transmitting the ultraviolet light into the processing chamber 101, such that the window 101C exposes the substrate 102 to be processed on the susceptor 101A. Further, outside the window 101C, there is provided an ultraviolet source 103 which is movable along the surface of the window 101C.
A silicon substrate as the substrate 102 to be processed is introduced into the processing chamber 101 shown in FIG. 21, and an oxidizing gas such as oxygen is introduced after vacuum evacuation to depressurize the inside of the processing chamber 101. Further, by activating the ultraviolet source 103, active radicals 0* are formed in the oxidizing gas. Such radicals activated by the ultraviolet light oxidize the exposed surface of the silicon substrate 102 and, thereby forming an extremely thin oxide film with a thickness ranging from about 0.4 to 0.8 nm on the surface of the silicon substrate 102.
In the substrate processing unit 100 shown in FIG. 21, it is possible to form the oxide film with a uniform thickness by moving the ultraviolet source 103 along the optical window 101C.
Because the oxide film thus formed is obtained by employing the UV-O2 oxidation process, the oxide film contains little defects such as interface states and is suitable for the base oxide film provided underneath the high-k dielectric gate insulating film, as reported by Zhang, et al. (Zhang, J-Y, et al.; Appl. Phys. Lett. 71(20), Nov. 17, 1997, pp. 2964–2966).
As described above, the base oxide film provided underneath the high-k dielectric gate insulating film needs to be extremely thin, and it is realizable to form a base oxide film having a thickness of about 0.8 nm by using a UV-O2 radical substrate processing unit.
On the other hand, conventionally it is noted that when a metal oxide film which has a small number of covalent bonds, i.e., a low stiffness, is formed directly on a single crystalline silicon substrate which has a large number of covalent bonds, i.e., a high stiffness, an interface between the silicon substrate and the metal oxide film becomes kinetically unstable, so that defects can be formed. In order to overcome such a problem, it is proposed that an oxynitride layer having a single atomic layer of nitrogen introduced therein is formed as a transition layer at the interface between the silicon substrate and the metal oxide film. Further, it is considered that forming such an oxynitride film as a base oxide film for the high-k dielectric gate insulating film suppresses a mutual diffusion of metal elements or oxygen in the high-k dielectric gate insulating film and silicon in the silicon substrate to thereby effectively prevent a diffusion of dopants from an electrode. In forming such an oxynitride layer, there is proposed a technique for nitriding a surface of an oxide film by using a microwave excited remote plasma (see G. Lucovsky, Y. Wu, H. Niimi, V. Misra, and J. C. Phillips; Appl. Phys. Lett. 74(14), Apr. 5, 1999, pp. 2005–2007; and ninth embodiment of Japanese Patent Laid-open Application No. 2002-1.00627).
Meanwhile, deterioration in quality of a thermal oxide film due to organic contamination of the silicon substrate surface occurring before growing the oxide film was pointed out long time ago in conventionally performed formation of a gate oxide film by using silicon thermal oxide film (for example, S. R. Kasi and M. Liehr; J. Vac. Sci. Technol. A 10(4), July/August 1992, pp.795–801). As the gate insulating film becomes getting thinner, it becomes more important to take effects resulting from the organic contamination into account when performing a process.
However, generally in a nitriding process using the microwave, an extremely high vacuum level of about 1.33×10−1 to 1.33×10−4 Pa (10−3 to 10−6 Torr) is required. When nitriding at such an extremely high vacuum level, effects attributed to a small amount of impure material such as oxygen or water remaining in the processing chamber become non-negligible, so that the oxide film can be thickened in nitriding. When the oxide film is thickened in oxynitriding, the advantageous effects achieved by using the high-k dielectric gate insulating film are offset. As described above, it has been difficult to nitride the extremely thin oxide film stably and reproducibly without thickening by oxidation at such a vacuum level as can be easily achieved to be used in a general semiconductor process.